Option 1: Install software from OpenIndiana repositories via IPS

OpenIndiana uses the same package management system as Oracle Solaris 11, and Oracle is thus far continuing Sun's practice of providing well written manuals:

http://download.oracle.com/docs/cd/E19963-01/html/820-6572/index.html

The command line package manager is 'pkg'

Set a remote repository:

     sudo pfexec pkg set-publisher -O http://pkg.openindiana.org/dev openindiana.org

Search for a package:

     pkg search -pr git

Install a package:

     sudo pkg install git

The above is the "native" package management but some packages are missing or outdated.

Option 2: Install software from SmartOS repositories via pkgin

All Illumos based systems like OmniOS, OpenIndiana and SmartOS can use the repository from Joyent/OmniOS as well.
Main advantage is that you find there a lot of very up to date packages.

A list of available software: http://pkgsrc.joyent.com/packages/SmartOS/

If you want to install software via pkgin (installs every package to /opt), you need to:

1. add path to /opt/local where all software is installed:
   export PATH=/opt/local/sbin:/opt/local/bin:$PATH
   
   
2. install the bootstrap-loader:
   curl http://pkgsrc.joyent.com/packages/SmartOS/bootstrap/bootstrap-2013Q2-x86_64.tar.gz | gtar -zxpf - -C /
   
   
3. update repository database:
   pkgin -y update
   
   
4. install the needed package, example Apache 2.4.6
   pkgin -y install apache-2.4.6
   
   more: http://wiki.smartos.org/display/DOC/Working+with+Packages

Option 3: Compile yourself

You need a compiler like gcc; download the sources, switch to the folder and compile via:

configure
make
make install